Air pollution remains a serious public health concern, particularly for children who are more susceptible to the adverse effects of ambient air pollution relative to adults because of more time spent outdoors, higher incidence of outdoor physical activity, and still developing biological and immune system. A substantial body of literature has linked exposure to criteria pollutants (particulate matter, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead) with adverse health outcomes in children, however, much of this literature is based on cross-sectional studies and therefore it is not clear whether these relationships merely capture the influence of selection or unobserved confounders. Most studies are based on geographically- limited samples thereby limiting generalizability. In addition, there is limited research on pollution's effects on children's cognitive outcomes despite preliminary evidence identifying a potential biological link. In this study, we propose to use longitudinal data on a nationally representative sample of children in the U.S and quasi-experimental methods to estimate the relationship between ambient air pollution exposure during the elementary and middle school years on children's health and cognitive outcomes. Our study has three specific aims: 1. Describe the short and long-term exposure to ambient air pollution in a nationally representative sample throughout elementary and middle school years and identify individual, family, and contextual factors that predict exposure. 2. Estimate the relationship between ambient air pollution exposure and children's health and cognitive outcomes including respiratory health problems, general health status, cognitive learning disabilities, cognitive development, and school absences. 3. Examine whether the estimated relationships between air pollution and children's outcomes vary with observable child characteristics including gender, race-ethnicity, socioeconomic status, participation in outdoor activities, and health characteristics. The analyses will link individual level data from the Earl Childhood Longitudinal Study - Kindergarten Cohort (ECLS-K) to the Environmental Protection Agency's criteria pollutant data. We will examine children's outcomes from all waves of the panel starting in kindergarten (n=21,260) through 8th grade (n=9,725). Given the complexities of addressing the potential endogeneity of pollution exposure, we will undertake several well- established econometric approaches including propensity score weighting, instrumental variables estimation, and child fixed effects models. Our findings will provide important evidence regarding whether reductions in air pollution are linked to children's health and cognitive outcomes, and consequently whether we can expect future efforts to control pollution to do so.